This invention relates in general to an improved method and apparatus for Active Noise Reduction (ANR) and more particularly to an ANR system at the ear with communication capability. The invention is a novel Electronic Earplug which combines active and passive noise reduction, a bilateral transducer drive, and a shunt feedback control correction network.
The Electronic Earplug is more compact, lighter, and less restrictive and requires less power to operate than headphone-based ANR systems which represent the closest known prior art.
A novel compensation of the electro-acoustic ANR feedback loop, based on the bilateral transducer drive and/or the shunt feedback control correction network maintains a satisfactory stability margin while generating a local sound counter wave. This counter wave is an acoustical means of active reduction of ambient noise which penetrates a passive plug barrier.
Active noise reduction is a negative feedback system, a concept which dates back to the seminal U.S. Pat. No. 1,686,792 by H. S. Black, issued in 1928. The specific application of negative feedback to an electro-acoustical ANR system was proposed in U.S. Pat. No. 2,983,790 by H. F. Olson, issued in 1961 and is described in articles by Olson and May (1953), and Olson (1956) and by Bleazey (1962). Such an ANR concept is summarized in FIG. 1 and is described in detail later. The ANR system in FIG. 1 produces an acoustical counter wave emanating from a speaker 15 which, under ideal circumstances, nearly cancels the effects of ambient noise p.sub.n which penetrates to a summing microphone 18. Hence, the microphone location defines a zone of acoustical quiet.
The ideal circumstances, mentioned above, imply that there is no phase shift around the feedback loop. In general, in an electro-acoustical system, phase shift is not constant but increases with frequency. It is therefore not possible to keep a constant phase shift over any extended frequency band. At a frequency where a 90.degree. phase shift is reached, the ANR system in FIG. 1 no longer cancels noise. At a frequency where a phase shift of 180.degree. is reached, negative feedback changes to positive feedback, and with sufficient gain, the system becomes unstable. The source of this undesirable frequency-dependent phase shift is twofold: sound wave propagation delay and acoustical circuit transmission delay.
Sound wave propagation delay is caused by the finite speed of sound in air which is approximately 344 m/sec. The phase shift caused by acoustical wave propagation delay is proportional to the distance over which the propagation delay is measured and to frequency, and is inversely proportional to the speed of sound. In an ANR system where distance between speaker and summing microphone 18 is short, phase shift caused by propagation delay is small. This makes the application of ANR in the Electronic Earplug very attractive. For example, for a speaker-microphone distance of 1 cm, a 180.degree. propagation delay induced phase shift occurs at 17.2 kHz. For stability, the loop gain at 17.2 Hz frequency can be reduced well below unity without an adverse effect on speech communication which requires typically only a 200 Hz-4 kHz bandwidth.
With a speaker-microphone distance in the earplug of 1 cm or less, the overriding cause of instability is the phase shift caused by the acoustical circuit transmission delay. For example, as will be elaborated further in connection with FIG. 3, an acoustical circuit of a typical loudspeaker consists of a mass of the diaphragm, a compliance due to diaphragm and air support and a resistive component due to losses and acoustical radiation. The combined effect of the above acoustical circuit components is the diaphragm resonance which causes a 180.degree. acoustical phase shift, often in the midst of the frequency region critically important to communication. Without the methods of compensation described later, diaphragm resonance and other acoustical circuit effects seriously limit the effectiveness of ANR systems.
Prior art closest to the Electronic Earplug is the use of ANR in headsets or headphones; e.g., as described in a paper by Dorey, A. P., Pelc, S. F., and Watson, P. R.: "An active noise reduction system for use with ear defenders". 8th International Aerospace Symposium, Cranfield, Mar. 24-27, 1975 and also more currently in U.S. Pat. Nos. 4,455,675 (Bose & Carter), 4,494,074 (Bose), and 4,644,581 (Sapiejewski).
Prior ANR art, as represented in the references mentioned above, uses only cascade loop compensation, i.e., compensation circuits, such as represented by block 14 in FIG. 1, which are connected in series with other loop components. Present invention teaches an improvement of electro-acoustical feedback loop stabilization in the form of a speaker bridge circuit and shunt feedback control network, so that large loop gain can be used over a wide frequency range to provide high noise reduction while at the same time maintaining a satisfactory margin of stability.
Prior art of speaker compensation based on a bridge circuit is represented by U.S. Pat. No. 3,647,969 (Korn). A serious disadvantage of the bridge prior art is that it provides no means of automatically maintaining balance.
Prior art of amplifier compensation using shunt feedback control impedance across load for improvement of high frequency performance is reported by A. F. Arbel, Analog Signal Processing and Instrumentation, Cambridge University Press, pp. 133-138, 1980 and is reviewed by R. K. Jurgen in IEEE Spectrum. pp. 41-43, April, 1972.